Increased capacity in 5G mobile communications requires rolling out massive MIMO base stations with network and mobile terminals at both sub-6 GHz and mmWave frequencies. Dynamic beamforming and the absence of RF test ports on the devices being tested make over-the-air (OTA) measurement pivotal to 5G deployment. Fortunately, OTA testing solutions employing software and hardware near-field transformations are meeting the challenges.
5G new radio (NR) communication systems will increase the capacity of mobile radio networks using frequency bands in the sub-6 GHz region, called frequency range 1 (FR1) by 3GPP, and the mmWave range (FR2). New technological approaches selected by the industry and 3GPP promise greater bandwidth at lower operational expense.
In FR1, the main innovation effort is focused on the base station, with the enabling of massive MIMO techniques.1 4G systems use single-user MIMO, where the user equipment (UE) calculates the inverse channel matrix to extract separate data streams. 5G multi-user MIMO (MU-MIMO) shifts the complexity from UEs to the base station by using a pre-coding matrix. Here, each data stream is received independently by separate receivers. Beamforming with antenna arrays of 64 to 512 elements reduces interference to adjacent users using MU-MIMO. In addition to facilitating the adoption of MU-MIMO to increase capacity, beamforming has other advantages. Its lower energy consumption brings a reduction in overall network operating costs by targeting individual UEs with their assigned signals.
Communication systems in the FR2 range use large available bandwidths at frequencies around 28 and 39 GHz. The impact is more than 60 dB path loss at 1 m distance and large electromagnetic field absorption in nearby objects. As with FR1 systems, the solution is to employ antenna arrays and beam steering, improving the gain on both the mobile device and base station sides of the network.
Whether for FR1 or FR2, 5G deployment relies on the performance of highly integrated solutions combining modem, RF front-end and antenna. The challenge is to define new methods and setups for performance evaluation, as RF test ports tend to disappear and beam steering technologies require system-level testing. In this context, both antenna and transceiver performance criteria must be measured OTA: effective isotropic radiated power (EIRP), total radiated power (TRP), effective isotropic sensitivity (EIS), total isotropic sensitivity (TIS), error vector magnitude (EVM), adjacent channel leakage ratio (ACLR) and spectrum emission mask (SEM). Assessing these OTA raises the critical question of the required measurement distance. Antenna characteristics are usually measured in the far field. Using direct far-field probing and applying the Fraunhofer distance criterion (R = 2D2/λ), a 75 cm massive MIMO device under test (DUT) radiating at 2.4 GHz should be evaluated in a chamber with at least 9 m range length. Even a 15 cm smartphone transmitting at 43.5 GHz needs a 6.5 m testing distance. This distance is required to create a region encompassing the DUT where the impinging field is as uniform as possible and approaches a plane wave with phase deviation below 22.5 degrees, known as the quiet zone.
Research shows that actual far-field behavior in the peak directivity region can start much closer than the Fraunhofer distance.2 These results proved, for example, that the far-field EIRP or EIS of a 15 cm DUT radiating at 24 GHz can be assessed at a distance as short as 1.14 m. Distance reduction of about 70 percent comes at the price of increased longitudinal taper error, caused by the deviation of the apparent phase center from the center of the measurement coordinate system. Also, sidelobe levels cannot be evaluated accurately at shorter distances.3 While direct far-field measurements at shorter distances are not convenient for all applications, there is an incentive to do so when conditions of application are verified. This is because large OTA anechoic chambers have high costs of ownership and limited dynamic range. Typical applications may be in the “white box” case, where the antenna location within the device and its aperture size are known.
NEAR-FIELD TO FAR-FIELD
Direct far-field measurements under “white box” assumptions may be inappropriate when the radiation aperture is larger than the quiet zone, the antenna cannot be precisely identified within the DUT or multiple antennas transmit simultaneously, e.g., from two extreme edges of a DUT which does not fit within the quiet zone. The “black box” scenario must then be considered, where the radiating currents can flow anywhere within the DUT. A first efficient approach to treat such cases in a compact environment is to employ software near-field to far-field transformations (NF-FF), for which the quiet zone size question becomes irrelevant. Mathematical implementations of NF-FF may vary, but the concept is generally the same: at least two polarization components of the electromagnetic field (E, H or a mixture of the two) are measured in magnitude and phase over a surface encompassing the DUT. The measured data is processed using functions to propagate the fields toward larger distances and extract far-field radiation components. From the Huygens principle, the knowledge of two phasors is enough to reconstruct exactly all six field components outside the surface. Alternative transformation methods use spherical wave expansion, plane wave expansion or integral equation resolution, with techniques to improve computational efficiency or accuracy by taking parameters such as spatial sampling rate, scanning area or truncation into account.
Figure 1 shows a commercial system capable of both direct far-field and near-field measurements with spherical scanning around the DUT using a conical cut positioner. On this system, the DUT is positioned on a turntable rotating in azimuth, while a dual-polarized Vivaldi antenna is mounted at the tip of a boom rotating in elevation. An RF test port available at the DUT connects one port of a vector network analyzer (VNA); the measurement antenna ports connect to two other terminals of the VNA, enabling near-field assessment through measurements of complex S-parameters.
Near-field measurement methods often rely on underlying assumptions about passive or RF-fed antenna testing:
- The antenna feed port is accessible with a signal fed to the antenna that is used as a phase reference.
- The RF signal is a continuous wave signal.
- Reciprocity applies so that transmit (Tx) and receive (Rx) patterns at the same frequency are identical.
There are workarounds available in Tx cases where such assumptions do not apply. For example, techniques can address the case of a DUT transmitting a modulated signal with no access to the antenna feed port. Hardware and processing implementations to retrieve the propagation phase vary, for example using interferometric techniques or multi-port phase coherent receivers4 with the addition of a dedicated phase reference antenna. For systems like those in Figure 1, this antenna is typically attached to the azimuth turntable. Alternative approaches include phaseless methods when the phase information is retrieved from magnitude measurements.
However, the Rx mode is more complex. First, the reciprocity assumption does not apply to mobile phone and base station devices, as the Rx RF component chain is, in general, different from the Tx RF chain. For a DUT with no test port, the power available at the Rx input of the RF front-end generated by an impinging wave coming from the probe antenna (here used as the transmitter) cannot be straightforwardly predicted in the near field. In other words, it is not possible to isolate the intrinsic receiving properties of the DUT in the far field from near-field coupling effects resulting from the test setup. There is also no access to a phase reference, so the NF-FF software transformation becomes inapplicable. Therefore, EIRP can be evaluated accurately in the near-field using NF-FF software but not EIS.
TRANSCEIVER PERFORMANCE MEASUREMENTS
Another key question is the OTA evaluation of radio transceiver performance, such as EVM, ACLR or SEM. Software NF-FF approaches are designed for processing periodic portions of the RF signal (the carriers) that determine propagation. However, this part of the signal is of no interest to assess these performance parameters, so the challenge is to extract information from the carrier modulation.
The first difficulty is that these quantities depend strongly on the signal-to-noise ratio (SNR) at the receiver (a spectrum analyzer in the Tx mode or the DUT in the Rx mode). This can be overcome by first assessing the complete 3D Tx or Rx pattern to determine the peak direction. Demodulation and EVM or other measurements can then be conducted at this specific location. The question remains whether the obtained values are reliable and reflect the results obtained in the far field. In the case of a single transceiver, the near-field EVM must be the same as the far-field EVM so long as the SNR is above a certain threshold dependent on modulation scheme, e.g., better than 20 dB. For multiple independent transceivers operating simultaneously, the near-field EVM may not be straightforwardly related to the far-field EVM because of positional dependence of the noise figure in the near field.